Conventional double heterostructure (DH) GaAs/Ga.sub.1-x Al.sub.x As diode lasers are generally observed to lase in several longitudinal modes simultaneously. Since each longitudinal mode is at a different wavelength this type of laser is not as useful as it might be in some applications, were its output single longitudinal mode. Specifically, all applications requiring coherence such as, for example, holography could be carried out with a single longitudinal mode diode laser, but not one operating multiple longitudinal mode. In addition, all optical communications applications including fiber optics are far better fulfilled by a single longitudinal mode laser.
Previous techniques for obtaining single longitudinal mode operation involve using wavelength selective elements either within or external to the laser. Gratings have been used internally (distributed feedback), at the ends (distributed Bragg reflection), and completely externally in conjunction with an anti-reflection coating on the crystal facet. Other techniques also use external frequency selective elements, but all of the above are difficult to implement, or are not viable economically, or increase the overall device size prohibitively, or degrade the operational diode lifetime. Thus, a simple solution to providing single longitudinal mode operation is needed.